Magnetic Resonance Imaging (“MRI”) is a highly useful technique for diagnosing abnormalities in biological tissue. Medical MRI requires creation of a substantially constant “primary” magnetic field, which passes through a patient's body. The patient is also exposed to electromagnetic waves in the radio frequency range, which vary with time in particular patterns. Under the influence of the magnetic fields and the radio waves, certain atomic nuclei within the patient's tissues resonate and emit other radio frequency waves, referred to as MR signals. Linear time varying “gradient” magnetic fields are typically superimposed on the primary field, based on the desired scanning sequence, to encode the MR signals in space and to select the desired image slice. An image of a volume of tissue is constructed with varying intensities corresponding to the concentration and/or physical state of certain nuclei of interest, by known mathematical techniques for correlating the magnetic field gradient patterns applied during the pulse sequence with the MR signals emitted by the patient. The concentrations or physical states of different substances ordinarily differ for differing kinds of tissues within the body, affecting the intensities in the image. Certain abnormalities in tissue, such as tumors, may be identified.
An MRI system is a highly complex, sensitive system. FIG. 1 is a schematic representation of a typical MRI system 10, comprising a magnet assembly with a gap region 50 (indicated schematically). A patient 11 is exposed to a primary magnetic field B within the gap region. A gradient coil system Gxt, Gy, Gz is provided in the gap region to establish linear, time varying gradient magnetic fields in three dimensions necessary for a particular scanning sequences. The gradient coil system is controlled by a gradient controller. Shimming coils (not shown) are also provided in the gap region to cancel non-uniformities in the primary magnetic field. A transmitter coil 26 for transmitting radio frequency pulses to the patient during the scanning sequence and a receiver coil 30 for picking up magnetic resonance signals during an actual scan are provided, as well. The same coil can act as both the transmitter and receiver coil, as is known in the art. The transmitter coil 26 is driven by an RF subsystem 24 though a variable amplifier 28. The receiver coil 30 is typically coupled to a pre-amplifier 32 and a variable amplifier 34, that amplify the received MR signals to an adequate level for further processing. A frequency down converter 36 is typically provided to shift the amplified signals, which are in a high frequency range, to a lower frequency range suitable for analog-to-digital (“A/D”) conversion by an A/D converting array 38. The A/D converting array 38 is coupled to a digital data processor 20, which filters and processes the data. The data processor 20 provides the processed data to a computer 14, which further processes the data for display by an image display system 16. An NMR sequence and timing controller 18 typically provides synchronization pulses to the major subsystems of the MRI system 10 to coordinate their operations. The computer 14 controls overall operation of the major subsystems of the MRI system 10.
Typically, the magnetic field strength in the patient receiving gap is greater than 300 gauss. 1 to 15 kilogauss are common. In addition to a strong magnetic field, medical MRI requires a magnetic field stability of the order of a few parts per million. One part per million or less is preferred. Deviations from proper performance can arise in any of the subsystems or components of the MRI system due to component degradation, power supply fluctuations, drift of analog components, environmental fluctuations, magnet drift and system or component failure, for example. In addition, it is often necessary to locate an MRI system in areas that have changing environmental magnetic fields, such as those generated by a third rail or the overhead wire of an electrical railway. Temperature fluctuations due to the changes in temperature during the course of a day or air conditioning, are also common environmental disturbances. In resistive magnets, the cooling water temperature is another source of temperature fluctuation.
Disruptions or drift in the primary magnetic field or the gradient magnetic fields, may cause ghosting, blurring and other artifacts in a resulting image. In severe cases, the imaging data may be completely destroyed. Considering the expense of an MRI procedure, as well as the discomfort of the patient, identification of deviations in the magnet and other problems in the system so that they can be corrected prior to conducting an imaging procedure is very important.
Images of test samples of material yielding a strong MR signal, such as petroleum jelly, water, salt water and nickel chloride, for example, have been used to identify deviations from desired behavior in MRI systems. The samples, referred to as phantoms, are placed in the imaging volume and subjected to a scanning sequence. The MR signals emitted by the sample are detected and compared to an expected response. Deviations from the expected response indicate a problem with the MRI system and may suggest where the problem is located. U.S. Pat. No. 5,432,449 discloses a test fixture comprising a mounting plate, a test coil and a receptor for a test sample. The test coil and the test sample are supported in a housing that fits within an aperture of the mounting plate. One or more spacer modules may be placed in the aperture to vary the distance of the test coil and the test sample from the mounting plate. The mounting plate is positioned on a table within the imaging volume for testing and is removed prior to imaging a subject.
U.S. Pat. No. 6,025,717, assigned to the assignee of the present invention, discloses a diagnostic simulator for evaluating individual subsystems of the MRI system, by selectively connecting the simulator to those subsystems of the MRI system. Data representative of a previously imaged object is provided to the selected subsystem in a proper form for processing by that subsystem. The data is also processed by subsystems downstream of the selected subsystem of the MRI system. An image may be constructed from the data and compared to a reference image, visually or by computer. By analyzing the results of the processing conducted by particular subsystems of the MRI system, problems may be isolated to a particular subsystem.
Other techniques for testing the MRI system include placing a probe in the gap region and implementing a pulse sequence. The probe may be connected to an oscilloscope or Teslameter to measure the magnetic field frequency and field strength.
One diagnostic program for evaluating an MRI system developed by Fonar Corporation (“Fonar”), Melville, N.Y., assignee of the present invention, is referred to as MSSR, or multi-slice scan reconstruction. In MSSR, digital data representative of raw magnetic resonance imaging data of a real or simulated phantom object, such as a cube filled with nickel chloride, is reconstructed. An image file is generated from the reconstructed data and displayed. The displayed image is viewed for errors, such as image artifacts, to identify problems in the display or data processor of the MRI device. Image artifacts indicative of a problem include banding, multiple images or fuzziness (ghosting).
Another diagnostic program developed and used by Fonar is incorporated into Fonar's ULTIMATE™ scanner. Raw digital image data from an actual scan of a phantom sample is loaded into memory and the data is processed for viewing. Again, the data processing and display systems can be evaluated. A fixed frequency can also be injected into the receiver coil and a scan performed. The resulting image should be a distinct dot. Ghosting in the image indicates a temporal instability somewhere in the system.